top: provide some protection against forking anomalies

This commit will eliminate a very nasty bug associated
with top's forest view mode.  It addresses a potential
SIGSEGV/SIGABRT that was only encountered when another
program (erroneously?) creates a lengthy forking loop.

If the growing list of nested children is sufficiently
fast such that proc_t start_time is duplicated between
children then the sort upon which top relies might not
produce the expected order. That, in turn, could cause
the forest_adds function to initially miss some child.

But that missed child would be caught by forest_create
and eventually would cause our array boundary overrun.
Such overrun occurs when some child of that originally
*missed* child is found and a duplicate add attempted.

In correcting this bug we'll also use this opportunity
to prohibit a borrowed proc_t padding byte (char) from
going negative. If the nesting level exceeded 127, the
effect was an "unnesting" with the snprintf width then
viewed as flag+width also yielding left justification.

Henceforth, we'll limit nesting to 100 with subsequent
children shown as " +  ", not the usual " `- " prefix.

References(s):
https://bugzilla.redhat.com/show_bug.cgi?id=1153642
http://www.freelists.org/post/procps/Bug-in-the-forrest-view,6

Signed-off-by: Jim Warner <james.warner@comcast.net>

top/top.h

@@ -780,7 +780,7 @@ typedef struct WIN_t {
 //atic void          keys_window (int ch);
 //atic void          keys_xtra (int ch);
 /*------  Forest View support  -------------------------------------------*/
-//atic void          forest_adds (const int self, const int level);
+//atic void          forest_adds (const int self, int level);
 //atic int           forest_based (const proc_t **x, const proc_t **y);
 //atic void          forest_create (WIN_t *q);
 //atic inline const char *forest_display (const WIN_t *q, const proc_t *p);